Counting Swaps: 5 Best Ways to Convert One Python List to Another

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πŸ’‘ Problem Formulation: In this article, we address the task of calculating the number of swaps necessary to transform one list into another, given two lists of equal length with distinct elements. For example, given the lists list1 = [1,2,3] and list2 = [3,1,2], the output should be the minimum number of swaps to turn list1 into list2.

Method 1: Greedy Swap Algorithm

The Greedy Swap Algorithm iterates through the list, swapping elements until the current list matches the target list. This method manually tracks swaps and stops when the lists are equal. Generally suitable for small lists because of its straightforward implementation.

Here’s an example:

def count_swaps(source, target):
    lookup = {val: idx for idx, val in enumerate(target)}
    source = [lookup[v] for v in source]
    swaps = 0
    for i in range(len(source)):
        while source[i] != i:
            swap_idx = source[i]
            source[i], source[swap_idx] = source[swap_idx], source[i]
            swaps += 1
    return swaps

print(count_swaps([1,2,3], [3,1,2]))



The code snippet defines the function count_swaps() which takes two arguments: the source list and the target list. It creates a lookup dictionary to identify target indices and performs swaps in a while loop until every element is in its target position. The swaps counter is incremented accordingly. The function returns the total number of swaps made.

Method 2: Count Inversions Using Merge Sort

This method involves modifying the merge sort algorithm to count inversions. An inversion corresponds to a pair of indices where the elements are out of order with respect to the final sorted sequence. This method takes advantage of the fact that the number of swaps can be equated to the number of inversions.

Here’s an example:

def merge_count_split_inversion(arr, left, right):
    count = 0
    merged = []
    while left and right:
        if left[0] <= right[0]:
            count += len(left)
    merged += left + right
    return merged, count

def count_inversions_and_sort(arr):
    if len(arr) <= 1:
        return arr, 0
    mid = len(arr) // 2
    left, inv_left = count_inversions_and_sort(arr[:mid])
    right, inv_right = count_inversions_and_sort(arr[mid:])
    merged, inv_split = merge_count_split_inversion(left, right)
    return merged, inv_left + inv_right + inv_split


([1,2,3], 2)

This snippet demonstrates a modified merge sort function count_inversions_and_sort() that recursively splits and counts inversions, using a helper function merge_count_split_inversion() to merge and calculate split inversions. The second element in the returned tuple represents the number of swaps.

Method 3: Cycle Detection

Utilizing cycle detection can pinpoint sequences within the list that rotate amongst themselves when transforming the list. The minimum number of swaps for a cycle of length n is n – 1. Summing these for each cycle yields the total swap count. This approach is efficient for densely packed swaps.

Here’s an example:

def count_swaps_cycles(source, target):
    visited = [False]*len(source)
    lookup = {v: i for i,v in enumerate(target)}
    swaps = 0
    for i in range(len(source)):
        if visited[i] or lookup[source[i]] == i:
        cycle_size, j = 0, i
        while not visited[j]:
            visited[j] = True
            j = lookup[source[j]]
            cycle_size += 1
        swaps += cycle_size - 1
    return swaps

print(count_swaps_cycles([1,2,3], [3,1,2]))



This code snippet employs cycle detection in the count_swaps_cycles() function. It constructs a lookup for the target indices, iterates through source elements, and skips processed elements or those already in place. It then traces cycles and counts their size, deducting one to get the needed swaps for that cycle. The total count is aggregated and returned.

Method 4: Simulated Annealing/Heuristic

Simulated Annealing offers a probabilistic technique to approximate the minimum number of swaps. It perturbs the order of elements iteratively and accepts or rejects the new order based on a temperature-dependent probability function. This is useful for large lists when an exact solution is expensive to compute.

Here’s an example:

import random

def simulated_annealing(source, target, iterations=1000, temp=100.0, cooling_rate=0.99):
    lookup = {val: idx for idx, val in enumerate(target)}
    current = source[:]
    current_swaps = count_swaps(current, target)
    for _ in range(iterations):
        temp *= cooling_rate
        i, j = random.sample(range(len(current)), 2)
        current[i], current[j] = current[j], current[i]
        new_swaps = count_swaps(current, target)
        if new_swaps < current_swaps or random.uniform(0, 1) < math.exp((current_swaps - new_swaps) / temp):
            current_swaps = new_swaps
            current[i], current[j] = current[j], current[i]  # revert swap
    return current_swaps

Output would vary due to the probabilistic nature

The snippet shows the simulated_annealing() function that introduces a random permutation to the order of elements and uses the count_swaps() from Method 1 to evaluate swaps. If the new order has fewer swaps or meets the temperature-dependent probability criteria, it is accepted; otherwise, the swap is reverted. This process repeats over several iterations until a satisfactory swap count is approached.

Bonus One-Liner Method 5: Approximation with Python Libraries

Python libraries such as NumPy or SciPy may offer functions approximating the answer, though not out-of-the-box for this specific problem. Using multidimensional array operations or optimization functions with custom heuristics can lead to an efficient approximate solution.

Here’s an example: (This is hypothetical and does not represent a specific function from the libraries)

import numpy as np

def count_swaps_numpy(source, target):
    source_indices = np.argsort(source)
    target_indices = np.argsort(target)
    return np.min(np.abs(source_indices - target_indices))

print(count_swaps_numpy([1,2,3], [3,1,2]))



This hypothetical one-liner employs NumPy’s powerful indexing to sort the elements and calculate the minimum number of swaps needed by using array operations. However, this example is for illustrative purposes only; real-world application would require a more complex implementation to accurately solve the problem.


  • Method 1: Greedy Swap Algorithm. Straightforward and easy to understand. Inefficient for large lists due to its O(n^2) time complexity.
  • Method 2: Count Inversions with Merge Sort. More efficient with a time complexity of O(n log n), but requires a modification of merge sort which is not intuitive for all programmers.
  • Method 3: Cycle Detection. Highly efficient for lists containing cycles. It requires a good understanding of graph theory but yields a quick performance.
  • Method 4: Simulated Annealing. Good for approximation on large datasets. Probabilistic and may not always provide an exact answer, but is fast for complex or large lists where exact solution is impractical.
  • Bonus Method 5: Approximation with Libraries. Leverages powerful array operations for approximation. May not provide exact solutions and can be complex to implement for this specific problem.